The invention deals with an arrangement of bioelectric electrodes comprised of an electrode material which may be connected to metallic conductor lines and which consists of an electrically conductive material containing a metal and/or carbon and of an ionic-conductive contact-electrolyte layer attached to one side of the electrode intended for a placing onto the skin.
Arrangements of bioelectric skin-contact-electrodes of this kind, have been known as for instance described in U.S. Pat. Nos. 3,987,055 and 3,933,049 as well as in "Medical Electronics", Oct. 1978, pp. 65-67. These electrodes are used for measuring biological potentials for instance in the fields of electrocardiography, electroencephalography, etc. and their functionality is based on electrochemical reaction taking place at the interfaces between the skin and an electrolyte at one side and between the electrolyte and an electron-conducting material on the other side. The materials from which the electrodes and the electrolytes are prepared, are to be selected in such a way to permit reversibility of the electrochemical reactions. A known electrode is prepared by using silver as the electrode material coated at its surface with silver chloride. A NaCl-gel is used as electrolyte.
The interfacial areas of the electrodes react as electrochemical half-cells. Consequently, in practical applications, with conventional electrode arrangements, substantial interference DC-voltages (offset voltages) are found superimposed over the bioelectrical signals. External, electrical compensation measures turn out to be very difficult since the interference voltages vary mostly considerably. Voltages externally applied to the electrode arrangement, will result in separations of charges and, therefore, in correspondingly high interference voltages which will only gradually decay after a relatively long recovery time.
In reference to conventional electrode arrangements, the electrolytes commonly used, will produce motion-dependent, variable artifact-voltages. Furthermore, the electrolytes used may also cause various skin irritations.
The objectives of the invention deal with the development of a bioelectric electrode assembly by which no or almost no interference voltage will interfere with the bioelectric signal. Moreover, the electrode arrangement is to be skin-compatible and is not to cause any skin irritations.
Starting from a bioelectric electrode arrangement as described above, the objectives have been achieved according to this invention whereby the contact-electrolyte layer and at least the adjacent areas of the electrode contain iodine ion-forming substances and/or whereby the contact-electrolyte layer contains an iodine ion-forming salt with a cation derived from the metal in the electrode material, present at least in the superficial areas adjacent to the contact-electrolyte layer.
The iodine containing substances of the contact-electrolyte layer have a disinfecting effect on the skin. The particular formation of the interfacial regions will assure low offset DC-voltages.
In regard to a preferred form of execution which may also be applied in electrode systems other than the described electrode arrangement, the base electrode consists of two areal opposed electrode layers of metal and/or of a carbon containing electrically conductive material whereby the two electrode layers are separated by an ionic-conductive electrolyte intermediate layer. The first electrode layer is attached to the connecting wires and the second electrode layer is coated at the opposite side away from the first electrode layer with a contact-electrolyte layer intended as the contacting surface with the skin. The electrolyte intermediate layer preferably containing also an iodine ion-forming substance and suitably having the same quantitative or at least the same qualitative composition as the contact-electrolyte layer, will assure a considerable reduction of the interference voltages of the electrode assembly. Besides, the fluctuations of the interference voltages are reduced. Furthermore, an electrode arrangement of this type has a relatively low AC-impedance and the recovery time, for instance after a simulated defibrillation, is relatively short with this type of electrode.
The electrode layers are preferably to contain the same metal in each layer, for instance silver, zinc or copper. In general, metals with an atomic weight of at least 51 are suitable. The intermediate electrolyte layer is preferably to contain a salt of this metal; more specifically, an iodine ion-forming salt of this metal. Therefore, bioelectric currents will be able to affect a reversible motion of the metal ions between the two electrode layers as well as a motion in both directions of the iodine ions between the second electrode layer and the two electrolyte layers. This will result in extremely low and negligibly fluctuating offset DC-voltages.
The intermediate electrolyte layer and the contact-electrolyte layer may have a liquid or gel-like consistency. These electrolyte layers may be absorbed in carrier substrates, such as absorbing paper or fleece or similar materials. The electrolyte layers, in particular the contact-electrolyte layer, may also be in a solid form. In this case, the powdery components of the electrolyte layers are mixed and pressed or sintered under high pressure. The use of solid electrolyte facilitates the handling of the electrode assembly.
Materials useful as iodine ion-forming substances are for instance iodides, in particular iodides of the metals used in the electrode layers, or triiodomethane (CHI.sub.3). For the purpose of minimizing a polarization of the electrode, germanium or a tungstate of the metals used in the electrode layers may be added to the electrolyte layers.
For the purpose of reducing the transfer resistance at the interfaces, salts of the metals used in the electrode layers may be added to the electrode layers. Suitable salts are in particular the iodides and selenides of these metals.
In regard to the quantitative composition of the electrode layers and the electrolyte layers, it is important that the major component by weight in the electrode layers is the metal, for instance silver, zinc or copper, while the major component by weight in the electrolyte layers is the iodine ion-forming substance.
The principle described in the foregoing whereby an intermediate electrolyte layer is placed between two electrode layers, may not only be applied if a solid contact-electrolyte layer is present but also in the presence of a gel-like contact-electrolyte provided that this electrolyte is capable of forming iodine ions.
Another form of execution deals with an electrode assembly whereby the electrodes contain carbon in the form of graphite or carbon black as the electrically conductive component. The carbon may be blended and molded with a synthetic resin, for instance an epoxy resin. Conventional electrode assemblies of this kind exhibit also very high offset DC-voltages. According to this invention, the offset DC-voltage can also be substantially lowered if an iodine ion producing substance, in particular triiodomethane (CHI.sub.3), is admixed to the carbon. Other suitable substances are metal iodides or iodo acetate I(CH.sub.3 CO.sub.2).sub.3 or iodo perchlorate. Suitable substances for the contact-electrolyte layer, are iodides as for instance zinc iodide or copper iodide as gels or also in solid forms. Also suitable are potassium iodide or calcium iodide.